Design of a new computer anti-theft system

In daily life, laptops are easy to carry, convenient to use, and expensive. If a computer is stolen or lost, it will not only cause loss of personal property, but also leak personal information or privacy, causing many inconveniences to oneself and others. Analyzing the computer anti-theft systems currently on the market, they have many defects such as large size, easy to be interfered by the outside world, and easy to be stolen.

Bluetooth is an open short-range wireless communication technology specification that works in the globally common 2.4GHz ISM band. The data rate of Bluetooth is 1Mb/s, and it performs frequency hopping at a rate of 1,600 hops/s on the globally open 2.45GHz ISM band. It uses the 2.4GHz ISM band without the need to apply for a license. Bluetooth itself has the characteristics of good confidentiality, strong anti-interference ability, low power consumption, and easy integration, especially its consistent encoding and decoding. At present, Bluetooth technology has matured globally, and its short-range transmission and modularization are of great help to our design.

This design applies Bluetooth technology to computer anti-theft. Bluetooth technology is used to detect whether the protected computer is within the safe range. When the protected computer is stolen, the Bluetooth receiver detects that the computer is out of the safe range and activates the alarm, thereby playing the role of computer anti-theft.

1 System Hardware Design

Hardware Design Overview The hardware design of this system is mainly divided into two parts:

①Bluetooth signal generating part; ②Bluetooth signal receiving and processing part.

The signal generator is carried by the user and transmits a Bluetooth detection signal to the signal receiver.

The signal receiving part can be integrated into the computer due to its small size to detect whether the computer is within the specified range and decide whether to alarm.

The signal generation part of this system uses the 555 timer to realize the second pulse working mode. Let the transmitting part send Bluetooth signals in a cycle of one second to fully reduce power consumption.

When the distance between the user's computer and the signal generator is within the specified range (10 m in this system), the Bluetooth receiver can receive the Bluetooth signal, indicating that the computer is safe and the microcontroller does not enable the alarm operation.

When the user's computer is out of the specified range, the Bluetooth receiver cannot receive the Bluetooth signal, indicating that the computer has been stolen, and the microcontroller enables the alarm operation.

The signal generation part includes power supply module, signal generation module and 555 timer module.

The signal receiving part includes power supply module, signal receiving module, microcontroller module and alarm module. The hardware schematic diagram is shown in Figure 1.

2 Hardware Design

2.1 555 Timer Module

The 555 timer is a medium-scale integrated device that combines analog and digital functions. The 555 timer has a wide power supply voltage range and can work at 4.5V~16V with reliable performance. It only needs to connect a few resistors and capacitors to realize pulse generation and conversion circuits such as multivibrator, monostable trigger and Schmitt trigger. Its internal circuit is shown in Figure 2.

The working principle of the 555 timer is as follows: when the input signal is input from pin 1 and exceeds 2VCC/3, the trigger is reset, the output pin 3 outputs a low level, discharges at the same time, and the switch tube is turned on; when the input signal is input from pin 2 and is lower than VCC/3, the trigger is set, the pin 3 outputs a high level, discharges at the same time, and the switch tube is turned off.

2.2 Transmitter module power supply

The power supply of the transmitting module includes a lithium battery, a charging circuit and a voltage stabilizing filter circuit, which provide a 3.3V working voltage for the Bluetooth signal generating module. The 5V voltage of the computer USB interface can easily charge the lithium battery, and the overcharge and overdischarge protection circuit can keep the lithium battery working in the best state. When an external USB power supply is connected, it powers the Bluetooth signal generating module and charges the lithium battery at the same time; when there is no external power supply, the lithium battery powers the Bluetooth signal generating module. The LDO (low voltage dropout linear regulator) chip SPX1117-3.3 of Sipex is selected to stabilize the output voltage at 3.3V. The power module is shown in Figure 3.

2.3 Bluetooth signal transceiver module

The Bluetooth signal transceiver module uses a GC-02 Bluetooth module produced by Nanjing Guochun Electric Equipment Co., Ltd. GC-02 is a Bluetooth data transmission module designed and produced using CSR's Audio_Flash Bluetooth chip. It uses laser blind hole processing technology, patch design, compact size, and is most suitable for industrial data and voice transmission. It is mainly used in Bluetooth voice transmission, car Bluetooth, Bluetooth voice gateway, etc.

When the Bluetooth transmitter module is powered on, it automatically establishes communication with the Bluetooth receiver module. The Bluetooth transmitter module is shown in Figure 4.

When GC-02 detects the Bluetooth signal and establishes a connection, the PI03 terminal will output a high level, otherwise it will output a low level. This signal is connected to the microprocessor AT89C2051 to determine whether a link is established. The Bluetooth receiving module is shown in Figure 5.

2.4 Microprocessor module.

The microprocessor module is the minimum system with AT89C2051 as the core, including power-on reset, crystal oscillation, etc. The microprocessor module is shown in Figure 6.

When the signal receiving module detects the high and low level changes of the P1.2 port, it means that the computer has been stolen. The microprocessor uses the timer to generate a PWM signal to drive the alarm module of the P3.7 port to complete the alarm action.

2.5 Alarm Module

The alarm module consists of a buzzer and two composite tubes, which make the circuit have two states. The PWM signal generated by the microcontroller AT89C2051 makes the composite tube alternately in the amplification and holding states, which can well drive the buzzer and realize the alarm function. The alarm module is shown in Figure 7.

2.6 Receiver Module Power Supply

The Bluetooth signal receiving part can be integrated into the computer due to its small size. The SP6641B-5.0 chip from Sipex is used to increase the power supply voltage of the lithium battery to 5V to power the microcontroller and alarm module. At the same time, the 5V voltage of the USB interface can be used to conveniently charge the lithium battery. The boost circuit is shown in Figure 8.

3 Software Design

The system software is designed using C language. Whether the system alarms is mainly determined by the microprocessor. When the distance between the user's computer and the signal generator is more than 10 m, the signal receiving module cannot receive the Bluetooth monitoring signal or the received Bluetooth signal is not the user's own signal. At this time, the Bluetooth connection is disconnected and the microprocessor enables the alarm action. Once the alarm action is enabled, the user must reset the microprocessor before the alarm action can be released. The program design flow chart is shown in Figure 9.

4 Conclusion

This article introduces in detail the design and implementation of a computer security system based on Bluetooth technology. The system has the following advantages: ① The chip and resistors and capacitors are all SMD type, small in size and easy to carry; ② It uses pulse signals and has low power consumption; ③ The system has a wide range of applications and the cost of this system is low, which is within the economic affordability of ordinary users.